Matt Boudreaux of Star Wars Galaxies has written a dev journal which details the development of invisible items in the game.

Hello everyone. Matt "AdeptStrain" Boudreaux here to talk about some of the invisible items we added for Game Update 6. I choose to do a diary on the invisible items because they are a good example on how quickly things can change in our development schedule and how flexible the team has to be to respond to those ever-changing demands. So, with that in mind, let's begin!

It's no secret that the Appearance window (also known as the A-Tab) was a project that Teesquared and I had seen in EverQuest II back in the beginning of the summer and started to make plans to port that concept over to Star Wars Galaxies. Throughout the (albeit few) design meetings we had about Appearance Tab, no one really brought up the idea of invisible items. The system in EQ2 had no such items (until apparently recently), and my own internal prototypes seemed to do quite well without any special items.

Read more here.